Head to the employee's personal profile > Employment Details tab > select edit > change Employment Type field to "Probation".
Once probation has been chosen for the employment type, the employee is only able to apply for unpaid leaves.
If you have changed the employment type from full time/part time to probation, as an admin, you may still see the leave entitlement as this is for record keeping, but rest assured, the employee is only able to apply for unpaid leave :)
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